To do RSA examples, you need to pick distinct primes p and q.
Enter a number m < 1,000,000,000 in the following web
form. If m is prime, you will
be told; if not, you will be given
back the least prime factor of m.

Enter m:

23

Once you have p, q and n = pq, you need to pick
an encryption exponent e, and solve for the decryption
exponent d. The main thing is that gcd(e, phi(n))
must be 1, where phi(n) = (p-1)(q-1).
The next web form will be useful for this.
It computes gcf(a, b) and writes it
as a linear combination of a and b.
So let a = (p-1)(q-1), and keep picking b at random
until you get one where gcd(a, b) = 1.
Then the y in the solution ax + by = 1 that it gives
is your d. (If y < 0, use d = (p-1)(q-1) + y, instead
to get a positive value for d.)

Enter a:

Enter b:

The RSA crypto system involves computing powers mod n.
Given a piece of data i, to encrypt it, compute
i^{e} mod n. To recover i from the encrypted value
j, compute j^{d}.
Here is a web form to do such computations:
Just plug in the variables m, n and a, and you will get back
the value of m^{a} (mod n).